In recent years, with regard to electronic equipment which is decreasing in size, thickness and weight, needs for a printed wiring board having a high density are more and more increasing. In accordance with the above needs, an improvement of a metal foil to be used is also required for the purpose of forming a fine circuit. Conventionally, as a method of forming a circuit, there has been adopted a subtractive process in which a circuit is formed by etching a metal foil or a (semi) additive process in which a conductor layer is formed on an insulating layer by plating. In the subtractive process, however, for making the adhesive strength between a metal foil and a B-stage resin composition become fine, a metal foil having a mat surface with remarkable roughness and having a high adhesive strength to an insulating layer is used. For this reason, there is a problem that, when a fine circuit is formed, part of convex portions is apt to remain on a resin surface of a laminate under the influence of the roughness of the mat surface of the metal foil. When an etching time is extended for removing the convex portions completely, the circuit is over-etched and the location accuracy of the circuit and adhesive strength decrease. As a means for overcoming the above problems, a so-called low profile metal foil of which the mat surface roughness is lowered has been realized. However, the above metal foil is originally weak in adhesive strength. When this metal foil is applied to a metal-clad laminate using a high heat resistant thermosetting resin, etc., a problem concerning fine circuits is that adhesive strength is insufficient. This problem is an obstacle to development of a printed wiring board having a high density. Further, as an example of use of a metal foil with an adhesive, e.g., a copper foil with an adhesive, there are proposed a copper-clad laminate using a copper foil having a thin adhesive layer formed thereon (for example JP-A-8-216340) and a copper-clad laminate using a copper foil to which a semi-cured resin film is bonded (for example JP-A-9-11397). However, these copper-clad laminates have problems in view of the level of adhesive strength and heat resistance after moisture absorption so that further improvements are required.
In the (semi) additive method, a roughening treatment of an insulating layer is required before plating for increasing the adhesive strength between the insulating layer and a conductor layer (for example JP-A-2003-69218 and JP-A-2003-249751). In this case, the step of forming a circuit becomes complicated. In addition, when a fine circuit is formed, a problem is that the accuracy of the formation of the circuit is apt to decrease because of an increase in the roughness of a surface. Further, there is disclosed a production process of a printed wiring board comprising forming a high heat resistant insulating resin layer as an outermost layer of a core base material and forming a conductor circuit on the insulating resin layer (for example JP-A-2004-6773). This production process requires a roughening treatment of the insulating layer so that this process cannot overcome the conventional problems.